Magnetic resonance imaging (MRI) is commonly used to image the internal tissues of a subject. MRI is typically performed by placing the subject or object to be imaged at or near the isocenter of a strong, uniform magnetic field, B0, known as the main magnetic field. The main magnetic field causes the atomic nuclei (spins) that possess a magnetic moment in the matter comprising the subject or object to become aligned in the magnetic field. The spins form a magnetization that precesses around the magnetic field direction at a rate proportional to the magnetic field strength. For hydrogen nuclei (which are the common nuclei employed in MRI), the precession frequency is approximately 64 MHz in a magnetic field of 1.5 Tesla. If the magnetization is perturbed by a small radio-frequency magnetic field, known as B1 magnetic field, the spins can emit radio frequency (RF) radiation at a characteristic frequency. The emitted RF radiation can be detected and analyzed to yield information that may be used to produce an image of the subject or object. For purposes of the discussion herein, the term “object” will be used to refer to either a subject (e.g., a person) or an object (e.g., a test object) when describing magnetic resonance imaging of that “object.”
In practice, magnetic field gradients are also applied to the subject or object in addition to the main magnetic field. The field gradients are typically applied along one or more orthogonal axes, (x, y, z), the z-axis usually being aligned with the B0, and introduce spatially-distributed variations in frequency and/or phase of the precessing nuclear spins. By applying the radio-frequency B1 magnetic field and gradient fields in carefully devised pulses and/or sequences of pulses that are switched on and off, the RF radiation emitted can carry spatially encoded information that, when detected and analyzed, may be used to produce detailed, high resolution images of the subject or object. Various techniques utilizing both specific pulse sequences and advanced image reconstruction methods have been developed, providing new advances, as well as introducing new challenges.
Single-shot EPI (ss-EPI) is a popular pulse sequence for many fast imaging applications, such as functional imaging, diffusion imaging and perfusion imaging. In addition to its robustness against motion and high data acquisition efficiency, ss-EPI also features a low specific absorption rate (SAR), which is desirable especially for high-field imaging. As the scope of advanced imaging broadens, however, the limitations of ss-EPI have become increasingly evident. In addition to its high sensitivity to magnetic susceptibility variations and eddy currents, the use of a single shot imposes a constraint on maximal k-space coverage, which greatly reduces the achievable spatial resolution. This problem becomes more pronounced as the magnetic field increases, because the shortened T2* relaxation time narrows the usable sampling window.
A common approach to high spatial resolution is to employ multi-shot pulse sequences. In addition to resolution improvement, multi-shot EPI also reduces magnetic susceptibility artifacts and eddy current sensitivity. A major downside is that the sensitivity to motion is increased. An effective way to address the motion problem is to incorporate the Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) sampling strategy (1) into multi-shot EPI. The initial implementation by Chung et al. assigned the phase-encoding direction along the short axis of the PROPELLER blades (2). Since the velocity of traversing k-space (dk/dt) remains slow along the phase-encoding direction, off-resonance effects in each blade still lead to significant artifacts. Recently, Skare et al. developed a different strategy whereby the phase-encoding direction is switched from the “long axis” to the “short axis” of the PROPELLER blades (3). This design dramatically increases the phase-encoding bandwidth, thereby reducing image distortion. Either implementation, however, suffers from off-resonance effects as well as a phenomenon known as “gradient anisotropy” (4), originally observed in oblique EPI acquisitions (5-7).
With its immunity to off-resonance effects, FSE or turbo spin echo (TSE) has been widely used clinically. Among a number of multi-shot FSE techniques developed over more than two decades, FSE with PROPELLER sampling has attracted a great deal of attention in recent years (8). In addition to the desirable properties inherited from FSE, PROPELLER-FSE provides effective self-navigation because of its inherent over-sampling around central k-space. Compared to EPI-PROPELLER pulse sequences, however, FSE-PROPELLER is considerably slower.